Dose prediction accuracy of collapsed cone convolution superposition algorithm in a multi-layer inhomogenous phantom

Abstract

Purpose: Dose prediction accuracy of dose calculation algorithms is important in external beam radiation therapy. This study investigated the effect of air gaps on depth dose calculations computed by collapsed cone convolution superposition (CCCS) algorithm. Methods: A computed tomography (CT) scan of inhomogenous phantom (30 × 30 × 30 cm 3 ) containing rectangular solid-water blocks and two 5 cm air gaps was used for central axis dose calculations computed by CCCS in Pinnacle treatment planning system. Depth dose measurements were taken using a cylindrical ionization chamber for identical beam parameters and monitor units as in the depth dose computations. The calculated and the measured percent depth dose (PDDs) were then compared. The data presented in this study included 6 MV photon beam and field sizes of 3 × 3 cm 2 , 5 × 5 cm 2 , 10 × 10 cm 2 , and 15 × 15 cm 2 . Results: The results of CCCS were within ±1.4% in the first water medium. However, upon traversing the first air gap and re-entering the water medium, in comparison to the measurements, the CCCS under-predicted the dose, with difference ranged from -1.6% to -3.3% for 3 × 3 cm 2 , from -2.4% to -4.2% for 5 × 5 cm 2 , from -2.4% to -6.7% for 10 × 10 cm 2 , and from -1.6% to -6.3% for 15 × 15 cm 2 . After the second air gap, the CCCS continued to under-predict the dose, and the difference ranged from -3.2% to -3.9% for 3× 3cm 2 , from -2.4% to -5.6% for 5 × 5 cm 2 , from -2.3% to -6.0% for 10 × 10 cm 2 , and from -1.5% to -5.6% for 15 × 15 cm 2 . Conclusion: The CCCS under-predicted the dose in water medium after the photon beam traversed the air gap. Special attention must be given during the patient set-up since large air gap between the patient body and immobilization devices may lead to unacceptable dose prediction errors. Keywords:Collapsed Cone Convolution Superposition; Heterogeneity Correction; PDD; Pinnacle; Dose Calculation IntroductionThe significant advances in external beam radiation therapy (EBRT) such as beam delivery capabilities have improved the dose conformity and distributions. 1 The intensity modulation radiation therapy (IMRT) is an example of EBRT that combines several intensity modulated beams leading to the construction of conformal dose distributions. 2,3 Most recently, a novel radiation technique called volumetric intensity modulated arc therapy (VMAT) was introduced. 3,4 The VMAT systems can deliver a highly conformal radiation dose to the target by allowing the simultaneous variation of gantry rotation speed, dose rate and positions of multiple-leaf collimators (MLC). 3,4 Several authors have conducted the evaluation of dose calculation algorithms for external beam radiation therapy. [5][6][7][8][9][10][11][12][13][14][15][16] Rana et al. 16 investigated the dose prediction accuracy of Acuros XB algorithm and anisotropic analytical algorithm (AAA) for different field sizes and air gap thicknesses. The results from that study 16 revealed that dose predictions errors up to 3.8% for Acuros XB and up to 10.9% for AAA ...

Treatment planning validation for symmetric and asymmetric motorized wedged fields

“…Thus, the quality of the beam itself is of significant importance in the dose precision. These results agreed with Muhammad Maqbool et al 24 Our study showed that an increase in wedge angle led to increase in the difference, and this is an agreement with Nath et al 25 , Pasquino et al 26 and Momennezhad et al 27 In this study, dose calculations were performed in Elekta 3DTPS, and it is recommended to carry out similar studies for other dose calculation algorithms such as collapsed cone convolution superposition (CCCS) algorithm 28 , pencil beam convolution (PBC) 29 , Acuros XB 30 and anisotropic analytical algorithm (AAA) 29,31 .…”

Clinical dosimetric impact of Acuros XB and analytical anisotropic algorithm (AAA) on real lung cancer treatment plans : review

“…2 A number of studies have shown the limitation of photon dose calculation algorithms such as analytical anisotropic algorithm (AAA), collapsed cone convolution (CCC) algorithm, and pencil beam convolution (PBC) algorithm when complex geometry with inhomogeneity is involved along the photon beam path. [3][4][5][6][7][8][9][10][11][12] Both the AAA and CCC are based on the superposition/convolution method, which calculates the dose by superposition of dose kernels of primary and scatter components that are derived from the Monte Carlo (MC). The tissue inhomogeneity correction in superposition/convolution method such as in the AAA is done both in the beamlet direction and lateral directions.…”

Limitation of Pencil Beam Convolution (PBC) Algorithm for Photon Dose Calculations in Inhomogeneous Medium

“…The accuracy of dose calculation algorithms is essential for tumor control; otherwise, inaccurate dose estimation may lead to tumor recurrence or higher normal tissue toxicities. The results presented in this study and other studies [2,[6][7][8][9][10][11][12] reveal that dose calculation algorithms have limitations in predicting accurate dose when photon beam passes through the high and low density heterogeneity. Hence, during the computed tomography (CT) simulation and patient treatment, it is essential to avoid the high and low density materials in the beam path prior to beam entering into the patient body.…”

“…Those results [6] are consistent with the findings presented in this study. Several other authors have also reported dose prediction errors by dose calculation algorithms [7][8][9][10][11][12] when inhomogeneity correction is applied for photon dose calculations. Dose differences may also vary depending on the photon beam energy used for calculations.…”